Like most 13-year-olds, Jenny Wilson likes to go shopping with friends. Her athetoid cerebral palsy means that she has used a wheelchair for almost a decade, but she is capable of negotiating busy high streets. Yet Jenny's independence is under threat – not from her disability per se, but by a legal anomaly that means she breaks the law if she uses the wheelchair that best meets her needs.

The teenager from Chester, in Cheshire, has outgrown the electric wheelchair she got two years ago. The model she needs, which includes better steering and a motorised seat enabling the user to stand, weighs around 175kg. The 1988 Road Traffic Act not only bans children under 14 from using very technologically advanced wheelchairs – a "class three vehicle" weighing up to 150kg – but it classes wheelchairs heavier than 150kg as cars, which can only be driven legally by over 17s who hold a driving licence.

So Jenny can either change to what would be an inadequate wheelchair on her 14th birthday next month, or wait until she is 17 for the most appropriate one.

Jenny says she appreciates that the act was never intended to further impair disabled people's mobility, but while technology has advanced – modern wheelchairs get heavier with adaptations such as motorised seats, communication aids or oxygen cylinders – legislation has stood still. She wants the law amended.

Her family learned about the legal anomaly when Jenny applied for a wheelchair grant from the disability charity Newlife Foundation for Disabled Children. Although Jenny was eligible for a grant, Newlife could not approve it for the model she wants. "This stops me from being independent," Jenny says. "A new powered wheelchair means I could go into the town with my mates without relying on my mum to push me." The chair would also enable her to stand, which would be more practical at her mainstream school in Chester and reduce the isolation she feels among her able-bodied peers.

"What's holding Jenny back is an inappropriate chair – not her disability," says Jenny's mother, Lynda Wakefield. "The current chair is front-wheel drive and doesn't hold the cambers of pavements, so Jenny can jackknife down. The equipment that could make a difference to Jenny's life, she can't have without breaking the law. To disable our disabled community further by legislation is nonsensical."

In theory, wheelchairs can be bought privately and no one would know the age of the user but, in reality, most wheelchairs are paid for with grants from disability charities. Funding should come through primary care trusts (PCTs) but a 2009 report by the Muscular Dystrophy Campaign revealed that 50% do not fund the full cost of a powered wheelchair for a disabled child. The research suggests PCTs put the average cost of a powered wheelchair at £2,000 when the true cost is £17,500, and even £3,000 only buys the most basic model.

There is no estimate for how many young people are affected by the law, but with around 70,000 wheelchair-using children in the UK, according to disability charity Whizz-Kids, Jenny is unlikely to be alone in her predicament. Newlife, alerted to the legal quirk by a PCT contact, has refused 12 applications for wheelchairs this year alone as a result. While there are no known cases of an arrest for breaching the act, a charity cannot make grants for equipment deemed illegal.

Newlife voiced its concern about the anomaly last year in a government consultation on mobility equipment. The local transport minister, Norman Baker, says the government will respond "in due course". He adds: "Powered wheelchairs and mobility scooters are important lifelines which provide independence to people with mobility issues in their day-to-day lives. However, it is important we balance the safety of pedestrians and other road users with the mobility needs of users."

Sheila Brown, chief executive at Newlife, says: "We've got 21st-century equipment and 1980s law. This is an easy amendment for government to make. It will simply allow charities and statutory services to fully respond to need instead of a make-do-and-mend lesser alternative."